Restore RainBarrel.is_editable
Restore RainBarrel.is_editable

The code was mistakenly removed on August 5 2016, in c805dc8385dfe595be21379b5a8858c24bca8eb3

Connects #2877

maurizi/otm-core,maurizi/otm-core,maurizi/otm-core,maurizi/otm-core